1. Field of the Invention
The present invention relates to a heating crucible for use in an organic thin film forming apparatus, and, more particularly, to a heating crucible for use in an organic thin film forming apparatus that has a thin-layered heater on its external surface.
2. Description of the Related Art
In general, organic electroluminescent (EL) devices include an anode layer and a cathode layer which are formed on a substrate in a predetermined pattern orthogonal to one another, with a plurality of organic layers, including a hole transporting layer, an emissive layer, and an electron transporting layer, sequentially interposed between the anode layer and the cathode layer.
In an organic EL device having such a structure as described above, a vacuum deposition technique is widely known to be suitable for forming organic thin films such as the hole transporting layer, the emissive layer, and the electron transporting layer. The vacuum deposition technique involves loading a substrate, on which a thin film is to be formed, in a vacuum chamber of 10−6-10−7 torr and vaporizing or sublimating an organic substance contained in a heating crucible to deposit an organic thin film on the substrate.
In the vacuum deposition method, the heating crucible, as a container of the organic substance to be deposited and as a heater for vaporizing or sublimating the organic substance, directly affects the condition of the organic thin film deposited from the organic substance. For this reason, active research on the heating crucible has been conducted recently.
FIGS. 1 and 2 show two types of conventional heating crucibles widely used in an organic thin film forming apparatus. FIG. 1 shows an open-style heating crucible 10, including a main body 11 having a mouth 12 at the top thereof, and a heater 13 surrounding the outer wall of the main body 11. Examples of such an open-style heating crucible are disclosed in various documents. According to the disclosure of Japanese Laid-open Patent No. 2000-223269, a plurality of small heating crucibles having small openings are used in order to improve the uniformity of an organic thin film. Japanese Laid-open Patent No. 2000-12218 discloses a technique of improving the uniformity in thickness of an organic thin film by using a heating crucible with a heater equipped to contact the outer wall of the crucible, in which the deposition rate is detected and controlled to ensure stable film deposition for a long duration. Japanese Laid-open Patent No. 2000-68055 discloses a heating crucible with two heaters: one equipped to contact the external side and bottom walls of a main body, and the other one protruding from the bottom of the main body. Japanese Laid-open Patent No. 2000-160328 discloses a heating crucible capable of improving the uniformity of an organic thin film deposited, in which a thermal reflecting member is arranged facing a heater equipped to surround a main body.
However, practically, organic thin films deposited on a substrate using such open-style heating crucibles, that are fully opened upward, have poor uniformity. For this drawback, although improving film uniformity has been one focus of research related to open-style heating crucibles as in the above-described patents, there is a limitation due to their open structure. In addition, open-style heating crucibles consume a large amount of organic substance 14 and raise costs, so it is difficult to use them for the mass production of EL devices.
Another style of a heating crucible used in an organic thin film forming apparatus, which has a cover 25 for covering a mouth 22 of a main body 21, is shown in FIG. 2. In the heating crucible 20, a vaporized organic substance 24 comes out through a nozzle 25a formed in the cover 25 and is deposited as a film on a substrate. When this style of a heating crucible is used, less of the organic substance 24 is consumed, and a more uniform organic thin film can be deposited on the substrate.
An example of such a heating crucible with a cover is disclosed in Japanese Laid-open Patent No. hei 10-195639. According to the disclosure of this patent, a cover having a smaller diameter than the heating crucible is placed above and close to an organic substance contained in the heating crucible, so as to control the deposition rate with more ease. Many other techniques for improving the nozzle structure of the cover have been suggested.
However, such heating crucibles with covers have the following problems.
As shown in FIG. 2, since a heater 23 for heating organic substances 24 is formed only on the outer circumference of the main body 21 of the heating crucible 20, the temperature of the cover 25, particularly near the nozzle 25a, of the heating crucible 20 is relatively low. As a result, the organic substance 24 is recrystallized near the nozzle 25a while sublimating. The recrystallized organic substance 24 adheres to the inner wall of the cover 25, and finally clogs the nozzle 25a. This clogging phenomenon can be prevented by raising the temperature of the external wall of the main body 21. However, in this case, the organic substance 24 chemically changes during deposition, so that the properties of a resulting device degrade. Therefore, it is detrimental to heat the main body 21 above a particular temperature.
As another solution to the clogging problem, an additional heater can be mounted on the top of the cover 25. In this case, a vacuum chamber where the heating crucible 20 is placed is overheated by the heat emitted upward from the heater, and thus, a heat-resistant member needs to be additionally installed above the cover 25. As a result, the structure of the vacuum chamber becomes complicated. In such a heating crucible with a cover, it is difficult to install a thermocouple, especially inside the heating crucible, to measure the temperature of the heating crucible or the cover.
Furthermore, the heating crucibles described above have the following problems because they utilize heat conducted from the heater.
As an organic substance comes out from the heating crucible by being vaporized or sublimated, and is consumed more and more, a heat conduction area of the heater becomes smaller, so that there is a need to raise the temperature of the heater to vaporize or sublimate a constant amount of organic substance. However, raising the temperature of the heater causes the organic substance to thermally decompose. Especially for organic substances to be deposited as thin films for organic EL devices that vaporize and chemically change at similar temperatures, considerable careful attention is required when controlling the temperature of the heater.